Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions and/or improve fuel economy. An EGR system may include various sensors to measure and/or control the EGR. As one example, the EGR system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed during non-EGR conditions to determine the oxygen content of fresh intake air. During EGR conditions, the sensor may be used to infer EGR based on a change in oxygen concentration due to addition of EGR as a diluent. One example of such an intake oxygen sensor is shown by Matsubara et al. in U.S. Pat. No. 6,742,379. The EGR system may additionally or optionally include an exhaust gas oxygen sensor coupled to the exhaust manifold for estimating a combustion air-fuel ratio.
As such, due to the location of the oxygen sensor downstream of a charge air cooler in the high pressure air induction system, the sensor may be sensitive to the presence of fuel vapor and other reductants and oxidants such as oil mist. For example, during boosted engine operation, purge air may be received at a compressor inlet location. Hydrocarbons ingested from purge air, positive crankcase ventilation (PCV) and/or rich EGR can consume oxygen on the sensor catalytic surface and reduce the oxygen concentration detected by the sensor. In some cases, the reductants may also react with the sensing element of the oxygen sensor. The reduction in oxygen at the sensor may be incorrectly interpreted as a diluent when using the change in oxygen to estimate EGR. Thus, the sensor measurements may be confounded by the various sensitivities, and the accuracy of the sensor, and thus, measurement and/or control of EGR, may be reduced.
In one example, some of the above issues may be addressed by a method for an engine comprising: during boosted engine operation with exhaust gas recirculation (EGR) flowing below a first threshold, modulating a canister purge valve (CPV) and estimating a purge flow rate based on an output of an intake oxygen sensor responsive to the modulating, the first threshold based on a response time of the CPV. In this way, an EGR estimate provided by the intake oxygen sensor can be corrected for the purge flow content.
For example, during boosted engine operation when EGR is flowing and purge flow is enabled (e.g., the CPV is open), purge flow vapors may cause a decrease in the intake oxygen measured by the intake oxygen sensor. Therefore, when the engine is boosted and EGR is flowing, a CPV may be modulated and the purge flow rate may be estimated based on the output of the intake oxygen sensor during the modulating. Specifically, an engine controller may open and close the CPV at a set frequency. The frequency may be based on a determined fuel canister load and a sensitivity of the intake oxygen sensor. Additionally, before modulating the CPV, the controller may decrease the EGR flow rate below a threshold, the threshold based on the modulating frequency. Estimating the purge flow during the modulating includes determining a change in intake oxygen measured by the intake oxygen sensor during the modulating (e.g., the change in intake oxygen between open and closed positions of the CPV) and then converting the change in intake oxygen to equivalent hydrocarbons. The estimated purge flow rate may then be used to correct the output of the intake oxygen sensor for purge flow, thereby eliminating the effect of purge on the intake oxygen measurement and resulting in a more accurate EGR estimate. Specifically, an engine controller may adjust the output of the intake oxygen sensor by the learned change in intake oxygen due to purge (e.g., purge correction factor). The adjusted output may be the change in intake oxygen due to EGR alone and not purge. Thus, the resulting EGR flow estimate may be more accurate and be used to adjust the EGR valve to deliver the desired EGR flow.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.